The long-term goal of the proposed research is to understand molecular basis and functional impact of skin connective tissue aging. Skin, like all human organs, undergoes deleterious alterations as a consequence of the passage of time. Natural aging of skin is manifested primarily by thinning, largely due to loss of type I collagen in the dermis. Type I collagen is the most abundant protein in skin and confers structure, strength and resiliency. Age-dependent loss of collagen causes increased fragility and thereby makes skin more susceptible to bruising and impedes wound healing. Aging of the US population makes medical care of fragile skin a growing public health concern. In addition to being the largest human organ, skin is readily accessible for study. These unique properties of skin provide the opportunity study molecular mechanisms of aging in humans. The free radical theory of aging posits that natural aging is driven by cellular damage that results from oxidation by reactive oxygen species (ROS) that are generated as a consequence of aerobic metabolism. We find that ROS levels are elevated in aged human skin fibroblasts in vivo. Fibroblasts are the major cell type that produces type I collagen. In addition, we find that the TGF-/SMAD/CTGF axis, which is the major regulatory network that drives type I collagen production in skin, is impaired in aged human skin. This impairment results from decreased expression of SMAD3, which is a downstream effector of TGF- actions, and reduced expression of connective tissue growth factor (CTGF), which is a multi-functional protein that acts in concert with TGF- to regulate type I collagen expression. Furthermore, we find that mild, short-term oxidative exposure of primary cultured human dermal fibroblasts causes permanent cellular alterations that closely mimic those observed in fibroblasts in aged skin in vivo; namely, increased ROS, reduced SMAD3, reduced CTGF, and reduced type I collagen expression. Based on these observations, we hypothesize that increased ROS, reduces expression of SMAD3 and CTGF, which results in reduction of type I collagen production, in fibroblasts in aged human skin. We propose four Specific Aims to test this hypothesis: 1) determine age-related alterations of ROS, SMAD3, CTGF, and type I collagen production, in human skin fibroblasts in vivo, 2) determine the ability of topical anti-oxidant to reduce ROS levels, mitigate impairment of the TGF-/SMAD/CTGF axis, and induce type I collagen production, in aged human skin in vivo, 3) determine molecular mechanisms by which oxidative exposure reduces SMAD3, CTGF and type I collagen expression in human ski fibroblasts, and 4) determine molecular mechanisms by which CTGF regulates type I collagen expression. The results from the proposed studies will provide important insights regarding 1) the age of onset of human skin aging, 2) molecular actions of topical antioxidant, 3) mechanisms by which oxidative exposure regulates the TGF-/SMAD/CTGF axis, and 4) molecular basis by which CTGF cooperates with TGF- in the regulation of type I collagen expression.